USInternational

USInternational is a keyboard layout application that allows you to duplicates Windows type option key sequences that allows accented characters. This allows people to perform accent characters without trying to figure out the MacOSX keyboard option layouts for special characters.
Unfortunately, this utility that remaps the keyboard layout, only effects usual applications and does not effect certain games (which usually overload your keyboard mapping) or X11 applications.
USI is freeware for MacOSX.